If life is rare, then perhaps they fear destroying us (for their own religious reasons) or fear the reactions of their own society if they publicize their discovery of us. If life is common, then perhaps there is a standardized protocol among civilizations that prohibits them from making contact until certain conditions are obtained.
Perhaps galactic manners demand not making contact until invited to do so. Thus, we might not ever meet aliens until we send out a contact protocol for potential contactors to follow, lest there be a dangerous misunderstanding. After all, it's generally considered to be poor form (among humans) to show up in someone's house uninvited, and if even one other alien society has a similar convention, then a previous misunderstanding and interstellar war might have made the survivors more cognizant of that potential custom.
Et tu BAUT? Quantum mutatus ab illo.